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Alien Controllers are floating alien creatures. They oversee Vortigaunts and Alien Grunts, and also act as soldiers as part of the Xen military forces. They share many physical traits with other intelligent Xen life forms, especially the Nihilanth, and are related to the Vortigaunts as indicated by their vestigial chest-arms. Their head is disproportionately large and can peel open like a flower to reveal a seemingly mechanical prong, resembling the one seen in the Nihilanth's head. They appear to have three eyes and a vertical mouth similar to that of Alien Grunts.

According to Marc Laidlaw, Controllers are part of the Nihilanth's support network. Since they are partially controlled by it, they may have simply died out after the Nihilanth was killed.[3]

Alien Controllers appear to be floating in a meditation pose. They're highly maneuverable and move around a lot to avoid getting shot at. They have two forms of attack. At long range, they shoot a stream of small fireballs from their hands, similar to the Nihilanth's fireballs but weaker and are not too accurate. At close range, they shoot a single larger and more powerful fireball with homing capabilities from their opened head.

In Half-Life, Controllers are first seen when Gordon Freeman is about to travel to the alien world Xen via a teleporter. Gordon has to protect a scientist preparing the teleporter. In Xen, Gordon comes across a number of Vortigaunts working in the Mines which, despite being aware of his presence, are not aggressive towards him unless attacked first or they become startled by nearby gunfire. They do, however, resume their normal hostile behavior while in the presence of Controllers. The Nihilanth can summon Controllers for help in the final battle of the game.

In Opposing Force, Controllers are only seen in the teleporter scene from the original game. In Blue Shift, they appear when Barney Calhoun activates the triangulation device on Xen and try to stop him from returning back to Earth. In Decay, they appear and try to stop the players from resetting the dampening locks.

Alien Controllers were originally referred to as the Flying Vortigaunts Controllers.[1] Their design appears to be inspired by generic 1950s aliens with large heads. The uncorrected proof of Half-Life 2: Raising the Bar states that the design of the Controller's head derives from the cut Flocking Floater, but this may be an error. Early screenshots show that its loincloth was intended to be blue in color. The final model contains several unused animations, including block and throw sequences. Controllers were to have their own unique gibs. The textures for these gibs can be found in Half-Life: Source.